Synthesis gas (syngas) is composed primarily of carbon monoxide and hydrogen. It is used as an intermediate feedstock in numerous industrial processes for producing various hydrocarbons (for example methanol), and as a source of hydrogen. Various processes exist for producing syngas from carbonaceous fuels, such as steam reforming and partial oxidation of natural gas.
Chemical-looping processes also can be used to produce syngas from carbonaceous fuels. In such processes metal-oxide particles are oxidized and reduced in a closed-loop cycle, wherein at one point in the cycle the particles are reduced in the partial oxidation of a carbonaceous fuel to yield syngas products. Examples of chemical-looping processes are chemical looping combustion, chemical looping gasification, and chemical looping partial oxidation.
In chemical looping combustion, which can be configured for power generation, metal-oxide particles react with a fuel to produce carbon dioxide and steam, resulting in reduction of the metal-oxide particles. The carbon dioxide and steam produced in the fuel reactor are easily separable, which allows for sequestration of carbon dioxide. The reduced metal-oxide particles are then regenerated with an oxidant such as air and recycled back to the fuel reactor. The regeneration of the metal-oxide particles is a highly exothermic process and the heat produced can be used for power generation.
In chemical looping gasification, which can be configured to produce hydrogen, metal-oxide particles react with a fuel to produce carbon dioxide, carbon monoxide, hydrogen and steam, again resulting in reduction of the metal-oxide particles. The reduced metal-oxide particles are regenerated in two steps. In the first step, steam partially oxidizes the reduced metal-oxide particles while producing hydrogen. In the second step, the partially re-oxidized metal-oxide particles are further oxidized with an oxidant such as air and recycled back to the fuel reactor.
Chemical looping partial oxidation uses metal-oxide particles to oxidize carbonaceous fuels (e.g. natural gas, which is primarily methane) to produce syngas.
These processes are referred to as ‘chemical looping’ because the metal-oxide particles in each process are recycled and reused continuously in a closed-loop pathway in the system. That is, the metal-oxide particles first are reduced in order to oxidize another reactant in a reactor, and the spent or partially spent (i.e. reduced) metal-oxide particles from the reactor are then re-oxidized in another part of the process before being redirected back to the reactor to be used again. Improved chemical-looping processes to produce syngas are of significant commercial interest.